Genymotion is a fast third-party emulator that can be used instead of the default Android emulator.
It runs as a virtual machine on your computer, so you get much better performance, and it's much more like using a real device instead of the Android Emulator.
You can download and install the Genymotion Personal Edition to explore the possibilities of this tool. Payed plans are available with more features.
Because it runs as a virtual machine you also need to install the free VirtualBox from Oracle.
Make sure Genymotion is running before you start your app. If you don't have a physical device attached, your NativeScript app will start in the Genymotion simulator with the command above.
You can even drop an APK into the simulator and test your app.